1. Field of the Invention
The invention pertains to a backup and archiving system by means of tape cassettes for data processing systems.
2. Description of the Related Art
At present, tape cassettes are the lowest-priced archiving medium for realizing backup and archiving systems. It is noteworthy that, on the one hand, the current significant growth in data volume entails an increasing number of tape cassettes (hereinafter also referred to as volumes) addressing one host unit individually. On the other hand, the growth in data volume entails only a limited increase in data volume per volume.
In addition, the principal use of backup and archiving systems currently takes place in an ever narrowing archiving window, since it must not impede any application operation. Thus, any backup and archiving system has to meet high requirements of parallelization in order to be able to transfer a volume of data.
It is noteworthy that the actually usable transfer rates for backup and archiving processes are, at present, markedly lower than those which tape technology can currently support. On the one hand, this is due to the fact that only limited data rates can be transferred by individual applications. Another reason is the fact that, whenever the data streams of several applications are clustered, access to the disk systems is constrained by the data structures of the systems platforms.
Moreover, tape systems have short innovation cycles for increasing capacity and a rate of transfer increasing by factors. One of the reasons is that more tracks are used to record on one tape. Another reason is that the degree of data compression is increasing.
Furthermore, manual operation of peripheral tape devices is more and more being automated by robot systems called stackers and ATLs (Automatic Tape Libraries).
Finally, a greater tendency to centralize archiving in computer centers is discernible, in an effort to do so in clusters of several platforms.
The above-described development trends bring about the problems detailed below.
For example, the average recording quantity (filling ratio) of the tapes decreases. Studies have revealed that, on average, less than 20% of the tapes are filled. Compounded with respect to the new technologies, this filling ratio threatens to drop to as little as 1%. The volumes are therefore filled only partially, and therefore again uneconomically.
The transfer rates of the cassette drives are not fully used and represent an unused potential.
The number of tapes/cassettes rises disproportionally, requiring appropriate shelf space, and leads to high cost.
Increased cost for shelf space in a robot, compared with a conventional shelf, exacerbates the cost problem.
The cassette drives are only fully used in phases; in other words, their use is uneconomical.
Since a high degree of parallelization is required for short-term peak loads, the number of cassette drives must, in addition, be increased even though the drives are rarely in use. In other words, the cost of investment is additionally increased by technical exigencies.
In today's tape technology, obstructions continue to occur during operation, brought about by extended mount and positioning times. For example, such obstructions may occur during a Reclaim, so called, of archived data for the purpose of reading or update.
As a rule, it is not possible to make adjustments to the host systems for optimal use of the tape and drive technology because functional adjustments within the applications are too costly.
Due to the incompatibility between various manufacturers and between generations of drives, problems arise to the user when expansion becomes necessary and when new technologies are used.
Previous approaches to solving these problems may be divided into two categories. One category pertains to individual solutions, while the other category pertains to integrated solutions.
A first individual solution may be referred to as “ATL” (Automatic Tape Libraries). ATL systems enable manual operation of the tape devices to be automated. Besides reducing the need for manual labor, the operation becomes more dependable and safer, and mount times are shortened because they proceed mechanically. Due to centralization for reasons of cost, an ATL is typically used jointly by several hosts. Thus, tape cassettes may frequently be used jointly by several systems regardless of the systems platform.
Another individual solution may be described as “virtual volumes”. In this solution, several volumes, viewed by the host as independently named volumes or cassettes, are embodied on one single physical volume. This increases the storage capability of the physical volumes (tape cassettes), so that fewer tape cassettes need to stand by. The specific properties of a new drive and the volumes run on it are no longer visible to the host. Therefore, adjustments no longer are necessary for operating the host, since the adjustments are captured by virtualization whenever transition is made to a new generation. In other words, the adjustments are accomplished in a software-driven virtualization stratum.
Yet another individual solution is temporary storage of data. The data are temporarily stored in a volume cache. In other words, entire virtual volumes are saved in a disk memory, in order to allow immediate writing (without prior mount time) and faster reading (without mount and positioning times). Retrieval and storage from a virtual volume to the volume cache may then take advantage of the physical tape transfer rate. In other words, the performance requirements of archiving may be met with fewer drives than in instances in which the host accesses the tape cassette drives directly. At the host interface, the number of available virtual cassette drives may be greater than available physically installed cassette drives.
Variable mechanisms are used for optimized management of the temporary disk memory, so that advance reservations etc. are possible. They control the exact time of secondary data transfer, i.e. the point at which a virtual volume is transferred between volume cache and physical cassettes.
In order to prevent data losses when errors occur in temporary memory, steps are taken to make the disks failsafe. The use of RAID disks, so called, is an example of such steps.
Volume caching is typically superimposed on a standard file system. A UNIX file system is an example of such a standard file system. This file system also contains data such as label contents from the virtual volumes being managed, as well as meta information. Meta information may, for example, be information indicating which virtual volume resides in which physical tape cassette, etc.
An appropriately dimensioned volume cache may, given a short archivation time window such as a 2-hour time window, achieve a nearly continuous optimal load for the cassette drives over 24 hours with high data traffic between host and disk.
In an integrated solution, virtualization, caching and operation are simultaneously achieved in any system via an ATL. Appropriate processing capacities are autonomously accomplished by a system of architecture visually presented in FIG. 1 in detail and hereinafter referred to as Architecture Model M1.
The integrated solution in accordance with Architecture Model M1 arises as a natural approach to solve the problems described above. On the other hand, Architecture Model M1 leads to new problems.
The system in accordance with Architecture Model M1 may itself become a bottleneck in the case of certain configurations. The scaleability of any system is, therefore, already too narrow for current installations.
It is furthermore problematic that guarantees versus the host regarding transfer rates are by now possible only up to a point, due to the system's complex internal processes with reciprocal obstruction. Particularly, operational obstructions may arise due to internal systems reorganization processes when, for example, a tape containing few data is to be fully used again because of a cassette recycling.
The scant use of the tapes is due, on the one hand, to additional data being regularly added to the end of the tape while, on the other hand, invalid data can be so marked but cannot be deleted from the tape. The obstructed space on the tapes can only be recaptured by a reorganization, i.e. by selecting, temporarily storing and then writing the desired data to a reformatted tape. This entails an enormous additional burden on the CPU, especially, and on the bus system of the archiving systems, with the effect that overall performance of the systems declines even further.
It is also problematic that the system additionally represents a new danger. Additional efforts are required to avoid systems failure when individual components fail.
To avoid the bottleneck in the data transfer, multiprocessors and multibus systems are used. However, all of the data transferred between host and disks, and all of the data transferred between disks and tape cassettes have to be moved via the CPUs' working storage, as the data formats, such as the blocking of data, header information etc., differ between host, caching disk and tape cassettes. Therefore, it turns out that the rate of transfer to the CPUs' working storage is the limiting factor for the data transfer of the entire system. This is true in equal measure for multiprocessor systems.
This potential bottleneck limits the scaleability of systems following Architecture Model M1 and may force the user to operate several systems with consequently separate data quantities. This results in organizational problems for the user, such as the need for reorganizing his internal work processes.
It is a particular problem that it is not possible to guarantee transfer rates versus the hosts. Indirectly launched transfers between the volume cache and physical tape cassettes obstruct the data traffic between host and volume cache, since they both have to be reformatted via the working storage. Resulting fluctuations of the transfer rates available to the host may require for their avoidance a reserve capacity in the system which cannot otherwise be provided. Attempts to copy from one cassette drive directly onto another without going through the volume cache, while reducing load on the working storage, require an additional physical drive. This renders the external control of the internal optimization even more difficult.
Failsafe dual systems in accordance with Architecture Model M1 have not appeared on the market so far. Since they involve additional coordination efforts, they may also entail additional bottlenecks and control problems.